The present invention relates to a circuitry for as well as a method of high-speed reading of a video cell for a video pickup chip including a plurality of such video cells disposed in the form of a two-dimensional array, and a reading logic designed for imaging a high input signal dynamic volume onto a reduced output signal dynamic volume.
A video cell of the aforementioned type is described in the German Patent DE 42 09 536 and is capable of imaging a high input signal dynamic volume in the light signals to be detected with a reliably determined, preferably logarithmic characteristic curve onto an output signal dynamic volume.
There the video cell includes at least two MOS transistors which are so connected to the photosensitive element that the photosensitive element is connected to the first main electrode of a first MOS transistor and to the gate of a second MOS transistor, and that moreover the gate and the other main electrode of the first MOS transistor are short-circuited and applied to an invariable potential so as to achieve a logarithmic characteristic curve, with an output signal amplifier with a high input impedance being connected to the second main electrode of the second MOS transistor.
In sensor cells of the type described in the aforementioned prior art document, however, a reduced potential of signal modulation may occur under certain operating conditions, i.e. in the case of a rapid signal modulation of the picked-up image field at a low excitation level. The cause of this situation is the internal resistance of the logarithm-forming transistor, which is dependent on the conditions of illumination, together with the bandwidth of the internal transmission characteristics, which is a function of the operating conditions. The consequence are trailing traces which occur particularly in dark scenes with moving bright image points, which produce an inexpedient effect.
The present invention is based on the problem of providing a circuitry for as well as a method of high-speed reading of a video cell for a video pickup chip including a plurality of such video cells disposed in the form of a two-dimensional array, and a reading logic designed for imaging a high input signal dynamic volume onto a reduced output signal dynamic volume, in such a form that the aforedescribed trailing or retracing effect will not occur in dark recording scenes. In particular it should be ensured that the video cells described in the German Patent De 42 09 536 may be read out as rapidly as possible so that the video cells will be read immediately after each exposure operation and reset into a condition in which they are available for a repeated exposure operation.
The inventive solution is defined in claims 1 and 6. Any features improving the inventive idea are the subject matter of the dependent claims. The inventive circuitry for high-speed reading of a video cell for a video pickup chip as described in the German Patent DE 42 09 536 is so designed that a further MOS transistor of the same charge carrier type is provided, which is connected in parallel with the first MOS transistor, the so-called logarithm-forming transistor, and has a main electrode which is short-circuited to the first main electrode of the first MOS transistor whilst its other main electrode is short-circuited to the other main electrode of the first MOS transistor, and that a reset voltage pulse may be applied to the gate electrode of the further MOS transistor.
In accordance with the invention the circuit for high-speed reading is operated in such a way that a resetting phase follows the reading operation chronologically, in which reset phase a reset voltage pulse is applied to the gate of the further MOS transistor (Mr1), which is beyond the supply voltage of the transistor and causes an operating point to be set outside the weak-inversion range, which ensures a sufficiently rapid discharge of the internal node. The potential at the internal node of the video cell can thus be set to a value, within the reset phase, which corresponds to a minimum illumination intensity. The cell is hence available again for a repeated exposure.